The low level of seismic activity which ended the previous week continued during the early part of the week under review. Although visibility was poor for most of the week some clear views were obtained of the scar feature; these suggested that fresh magma was again getting close to the surface. EDM measurements on the northern triangle to Farrells were occupied during the week and continue to show a very slight shortening trend. Radial lines to the volcano at Amersham were also measured with no consistent trends being shown. The MVO GPS network was modified during the week with an expansion of the network to cover the entire island. Monitoring of the health aspects of the eruption has been improved during the week and an expanded program of air and particle sampling was started.
Low cloud cover and overcast conditions throughout the week did not allow any clear views to be obtained of the volcano. No clear views were obtained on 22, 23 and 24 September. The cloud base lifted sufficiently on 24 and 26 September to allow brief views of the scar, but for the remainder of the week only the flanks of the volcano and the scree slope in the Tar River valley were visible.
Rockfalls throughout the week have been very small, mainly concentrated within the scar and associated with continued stabilisation of the inner walls of the scar. Light ashfall, possibly associated with small rockfalls into the scar, was observed by a field team to Chances Peak on 28 September.
Improved visibility during late afternoon on 24 September allowed some brief views of conditions at English's Crater. There were no signs of new material at the bottom of the scar apart from debris derived from rockfalls off the side walls of the scar. Abundant steaming and sulphur deposits were observed at the base of the scar feature. Low cloud only allowed views of the lower flanks of the dome. Several unstable were present on the northern and eastern flanks of the dome.
Visibility was relatively good on 26 September, although the top of the dome remained obscured. Some red-hot rock and high temperature gases were seen in the bottom of the scar suggesting that fresh magma was getting close to the surface again; however material falling from the walls of the scar covered any new dome which might be forming.
The relatively quiet period which followed eruptive activity on 17-18 September continued during the early part of the week under review. Small rockfall and volcano-tectonic earthquakes dominated seismicity at the volcano during the week with an increase in the level of activity beginning from 26 September and increasing markedly at the end of the reporting period.
Earthquake types: 22 to 28 September 1996
These earthquake counts are made manually from the helicorder record for the Gages seismic station.
Date VT LP Hybrid RF Tremor 22/09 0 0 1 7 High 23/09 0 0 11 44 Mod 24/09 0 7 5 58 Low 25/09 4 1 12 81 Low 26/09 22 0 16 13 Mod 27/09 33 3 9 11 Low 28/09 69 2 17 49 Low
Small rockfalls were the most dominant type of signal recorded during most of the week with hybrid and volcano-tectonic activity becoming more prominent during the latter part of the week. Rockfalls during the early part of the week were caused by continued stabilisation of the steep inner walls of the scar produced by the 17-18 September activity.
Volcano-tectonic earthquakes which had been non-existent during the early parts of the week reappeared from 26 September onwards. The VT's recorded were somewhat transitional with hybrid events with a short high frequency onset and low frequency coda. The number of events increased towards the end of the week. The levels of long period and hybrid events remained comparatively low throughout this period averaging less than 11 events per day. Hybrid activity increased somewhat during the latter part of the week in tandem with the increase in VT activity.
The level of tremor on the Gages seismometer has been quite variable throughout the reporting period. Tremor levels were generally high during the earlier parts of the week due to heavy rain and thunder and lightning activity. Mudflow signals were recorded between 06:25 and 06:35 on 22 September and 06:34 and 07:33 on 24 September. These flows resulted from remobilisation of thick ash deposits in Fort Ghaut, from the 17-18 September eruption.
One regional event, of magnitude 5.2 (Mt) located at 152 km depth, 40 km from Martinique, was recorded by the seismic network at 07:42 on 24 September. The event was felt in Martinique, Antigua and St Lucia.
EDM measurements were again severely hampered during the early part of the week by low cloud cover and generally poor weather. The northern triangle to Farrells was occupied on 25 September and showed shortening of 4 and 11 mm on the St. Georges Hill to Farrells and the Windy Hill to Farrells line. Although there has been very little consistency in line length changes on this triangle there is a slight overall trend of shortening on these lines. Radial lines between Lower and Upper Amersham and Lower Amersham and Chances Peak steps were also occupied during the period. Line lengths between Lower - Upper Amersham and Lower Amersham - Chances Peak steps showed changes of + 48 mm and - 1 mm respectively between the period 20-26 September.
A GPS survey was completed at the beginning of the week on the new MVO network of stations on the volcano. A second occupation of this new network would be required before any changes in line lengths can be documented. Preliminary in-house processing of data collected before and after the 17-18 September eruption had indicated that there may have been significant deformation accompanying this eruption. Further processing of the data by independent sources now suggest that there was no significant deformation event associated with this eruption.
The GPS network operated by the University of Puerto Rico was reoccupied during the reporting period using stations at Hermitage, Chances Peak, Reids Hill and St Georges Hill. The results of these measurements are not yet available.
Some mapping of the new deposits in the Tar River Valley and on the pyroclastic flow delta was undertaken by the helicopter on 27 September.
The COSPEC instrument is still out of operation and SO2 tube analyses for this period have not been reviewed. The COSPEC instrument is expected to be returned next week and would be reintroduced immediately into the monitoring program.
A sampling expedition to the Tar River area undertaken on 22 September obtained a temperature of 373 degrees Centigrade at a depth of 45 cm in the pyroclastic surge deposits close to the Tar River Estate House.
An air and particle sampling network was initiated by Dr Peter Baxter and Andy Nichol on 23 September. The sampling will enable better characterisation of the ash and assessment of its health impacts.
Dr Angus Miller BGS
Dr Anne-Marie Lejeune, BGS
Bennett Simpson, BGS
Graham Ryan, MVO
LeVar Cabey, MVO
Visitors & Scientific Collaborators
Dr Peter Baxter, University of Cambridge
Mr Andy Nichol
Professor Steve Sparks, Bristol University
Dr Glen Mattioli, University of Puerto Rico
Dr Alan Smith, University of Puerto Rico
Lorna Nieves, University of Puerto
Rico Dr Jean-Jacques Jeremie, University of Guadeloupe